John Hageman's factor and deep-vein thrombosis: Leiden thrombophilia Study

High molecular weight kininogen interactions with the homologs prekallikrein and factor XI: importance to surface-induced coagulation

BACKGROUND

In plasma, high molecular weight kininogen (HK) is either free or bound to prekallikrein (PK) or factor (F) XI (FXI). During contact activation, HK is thought to anchor PK and FXI to surfaces, facilitating their conversion to the proteases plasma kallikrein and FXIa. Mice lacking HK have normal hemostasis but are resistant to injury-induced arterial thrombosis.

OBJECTIVES

To identify amino acids on the HK-D6 domain involved in PK and FXI binding and study the importance of the HK-PK and HK-FXI interactions to coagulation.

METHODS

Twenty-four HK variants with alanine replacements spanning residues 542-613 were tested in PK/FXI binding and activated partial thromboplastin time clotting assays. Surface-induced FXI and PK activation in plasma were studied in the presence or absence of HK. Kng1-/- mice lacking HK were supplemented with human or murine HK and tested in an arterial thrombosis model.

RESULTS

Overlapping binding sites for PK and FXI were identified in the HK-D6 domain. HK variants with defects only in FXI binding corrected the activated partial thromboplastin time of HK-deficient plasma poorly compared to a variant defective only in PK-binding. In plasma, HK deficiency appeared to have a greater deleterious effect on FXI activation than PK activation. Human HK corrected the defect in arterial thrombus formation in HK-deficient mice poorly due to a specific defect in binding to mouse FXI.

CONCLUSION

Clinical observations indicate FXI is required for hemostasis, while HK is not. Yet, the HK-FXI interaction is required for contact activation-induced clotting in vitro and in vivo suggesting an important role in thrombosis and perhaps other FXI-related activities.

Factor XII Deficiency in Mexico: High Prevalence in the General Population and Patients with Venous Thromboembolic Disease

INTRODUCTION

Thrombosis is one of the leading causes of morbidity and mortality worldwide. Venous thromboembolic disease (VTD) is considered a new epidemic. FXII deficiency is supposed to be a cause of thrombosis. To search for unknown causes of thrombosis in our population, our aim was to determine if FXII deficiency can be considered a risk factor for VTD.

METHODS

Young adult Mexican patients with at least one VTD episode and healthy controls were included in this prospective, observational, controlled study. Liver and renal function tests, blood cytometry, and blood coagulation assays were performed. Plasma FXII activity and its concentration were evaluated.

RESULTS

Over a two-year period, 250 patients and 250 controls were included. FXII activity was significantly lower in the control group compared to patients with VTD (p = 0.005). However, percentage of patients and controls with FXII deficiency was 8.8 and 9.2%, respectively (p = 1.000). No significant association was found between FXII deficiency and VTD (p = 1.0). FXII plasma concentration was lower in controls vs. patients with VTD: 4.05 vs. 6.19 ng/mL (p <0.001). Percentage of patients with low FXII plasma concentration was 1.6% and 6.0% in patients and controls, respectively (p = 0.010).

CONCLUSIONS

FXII deficiency is a frequent finding in patients with VTD and controls in Mexico. Some patients with FXII deficiency had normal APTT result, an effect not described above. FXII plasma concentration was lower in patients with low activity.

Clotting of the Extracorporeal Circuit in Hemodialysis: Beyond Contact-Activated Coagulation

Thrombotic complications in patients with end-stage kidney disease are frequent. While being a lifesaving treatment for these patients, hemodialysis introduces a thromboinflammatory environment. Additionally, the extracorporeal hemodialysis circuit itself is prone to clotting because of an interaction between different activation mechanisms of the coagulation system, platelets, and the immune system. Anticoagulation of the patient and the machine is frequently complicated by bleeding. We discuss the factors important in this balancing act and touch on potential strategies that are on the horizon to target thromboinflammation.

Factor XII deficiency evaluated by thrombin generation assay

INTRODUCTION

Coagulation factor XII (FXII) plays a role in thrombin generation, fibrinolysis, inflammation, angiogenesis, chemotaxis and diapedesis. FXII deficiency is not associated with bleeding risk unlike other coagulation factors.

MATERIALS/METHODS

We investigated thrombin generation assay (TGA) profile modification in FXII deficiency and the correlation with TGA and deficiency severity. TGA was performed in platelet poor plasma (PPP) with tissue factor (1 pmol/L) and phospholipid (4 µmol/L) standardized concentration. Thrombin generation profiles were compared in 54 patients with FXII deficiency, 25 healthy controls and 23 patients with hemophilia A (factor VIII (FVIII) deficiency. Patients with FXII deficiency were classified in three groups based on FXII activity (30-50%, 10-29%, <10%). FVIII deficiency was included as a bleeding control group.

RESULTS

As expected, we found a correlation between FXII deficiency and activated partial thromboplastin time (aPTT). A decrease of thrombin generation was observed in healthy controls and all FXII deficiency groups. A decrease of endogenous thrombin potential (ETP), peak and velocity was observed in patients with FVIII deficiency compared to FXII deficiency. A decrease of thrombin generation was noted in patients with FXII deficiency and bleeding history compared to patients with FXII deficiency and thrombosis history.

CONCLUSION

In this study, thrombin generation profiles were not sensitive to FXII deficiency. TGA could distinguish bleeding and thrombotic tendency in FXII deficiency. Our results should therefore be considered as exploratory and deserve confirmation.

Case Report: An Adult Patient With Deficiency of Adenosine Deaminase 2 Resembled Unilateral Frosted Branch Angiitis

Purpose: Deficiency of adenosine deaminase 2 (DADA2) is a rare autosomal recessive systemic autoinflammatory disorder. We describe a rare case of an adult patient with DADA2 who presented with unilateral frosted branch angiitis (FBA) combined with branch retinal vein occlusion and panuveitis.

Method: This paper is a clinical case report.

Results: A 31-year-old male patient complained of blurred vision in his right eye for 2 days. His fundus examination showed FBA combined with branch retinal vein occlusion and panuveitis. He had a medical history of intermittent and recurrent fever, skin rash and aphthous ulcer for 5 years, and lacunar infarction for 1 month. Laboratory examinations showed hypogammaglobulinemia and mild prolonged activated partial thromboplastin time (APTT). Brain magnetic resonance imaging (MRI) revealed old lacunar infarction in the right basal ganglia and the lateral ventricle and fresh lacunar infarction in the right pons, respectively. The perivascular sheathing of FBA and macular edema were resolved after steroid administration and treatment of intravitreal anti-VEGF injection. During the period of follow-up, the patient subsequently suffered from recurrence of strokes, abnormality of coagulation function, sudden hearing loss of the left ear, and diplopia. His gene sequencing results demonstrated several deletion mutations in ADA2, and the diagnosis of DADA2 was eventually confirmed.

Conclusions: FBA represents a very rare ocular feature of DADA2 and may in some cases be the presenting manifestation. Therefore, ophthalmologists need to be aware of this rare autoinflammatory disease.

The rebirth of the contact pathway: a new therapeutic target

PURPOSE OF REVIEW

Anticoagulation with vitamin-K antagonists or direct oral anticoagulants is associated with a significant risk of bleeding. There is a major effort underway to develop antithrombotic drugs that have a smaller impact on hemostasis. The plasma contact proteins factor XI (FXI) and factor XII (FXII) have drawn considerable interest because they contribute to thrombosis but have limited roles in hemostasis. Here, we discuss results of preclinical and clinical trials supporting the hypothesis that the contact system contributes to thromboembolic disease.

RECENT FINDINGS

Numerous compounds targeting FXI or FXII have shown antithrombotic properties in preclinical studies. In phase 2 studies, drugs-targeting FXI or its protease form FXIa compared favorably with standard care for venous thrombosis prophylaxis in patients undergoing knee replacement. While less work has been done with FXII inhibitors, they may be particularly useful for limiting thrombosis in situations where blood comes into contact with artificial surfaces of medical devices.

SUMMARY

Inhibitors of contact activation, and particularly of FXI, are showing promise for prevention of thromboembolic disease. Larger studies are required to establish their efficacy, and to establish that they are safer than current therapy from a bleeding standpoint.

Endovenous Thermal Ablation for a Varicose Vein Patient with Factor XII Deficiency: A Case Report

Factor XII (FXII) deficiency is a rare coagulation disorder, and its potential relationship with venous thrombosis was reported. Here we present a case of a 67-year-old woman with FXII deficiency who successfully underwent endovenous thermal ablation (ETA) for primary varicose vein due to the incompetent great saphenous vein (GSV). The FXII deficiency was revealed through preoperative examinations, and the patient underwent ETA as a day surgery. For prophylaxis of thrombosis, she received compression therapy alone. Her postoperative course was uneventful, without any kind of thrombosis. In the presence of FXII deficiency, ETA could be safely performed.

Annexin A4 inhibits sulfatide-induced activation of coagulation factor XII

BACKGROUND

Factor XII (FXII) is a plasma serine protease that initiates the intrinsic pathway of blood coagulation upon contact with anionic substances, such as the sulfated glycolipid sulfatide. Annexins (ANXs) have been implicated in the regulation of the blood coagulation reaction by binding to anionic surfaces composed of phospholipids and sulfated glycoconjugates, but their physiological importance is only partially understood.

OBJECTIVE

To test the hypothesis that ANXs are involved in suppressing the intrinsic pathway initiated by sulfatide, we examined the effect of eight recombinant ANX proteins on the intrinsic coagulation reaction and their sulfatide binding activities.

METHODS

Recombinant ANXs were prepared in Escherichia coli expression systems and their anticoagulant effects on the intrinsic pathway initiated by sulfatide were examined using plasma clotting assay and chromogenic assay. ANXA4 active sites were identified by alanine scanning and fold deletion in the core domain.

RESULTS AND CONCLUSIONS

We found that ANXA3, ANXA4, and ANXA5 strongly inhibited sulfatide-induced plasma coagulation. Wild-type and mutated ANXA4 were used to clarify the molecular mechanism involved in inhibition. ANXA4 inhibited sulfatide-induced auto-activation of FXII to FXIIa and the conversion of its natural substrate FXI to FXIa but showed no effect on the protease activity of FXIIa or FXIa. Alanine scanning showed that substitution of the Ca2+ -binding amino acid residue in the fourth fold of the core domain of ANXA4 reduced anticoagulant activity, and deletion of the entire fourth fold of the core domain resulted in complete loss of anticoagulant activity.

Plasma contact factors as therapeutic targets

Direct oral anticoagulants (DOACs) are small molecule inhibitors of the coagulation proteases thrombin and factor Xa that demonstrate comparable efficacy to warfarin for several common indications, while causing less serious bleeding. However, because their targets are required for the normal host-response to bleeding (hemostasis), DOACs are associated with therapy-induced bleeding that limits their use in certain patient populations and clinical situations. The plasma contact factors (factor XII, factor XI, and prekallikrein) initiate blood coagulation in the activated partial thromboplastin time assay. While serving limited roles in hemostasis, pre-clinical and epidemiologic data indicate that these proteins contribute to pathologic coagulation. It is anticipated that drugs targeting the contact factors will reduce risk of thrombosis with minimal impact on hemostasis. Here, we discuss the biochemistry of contact activation, the contributions of contact factors in thrombosis, and novel antithrombotic agents targeting contact factors that are undergoing pre-clinical and early clinical testing.

The prevalence of heterozygous F12 mutations in Chinese population and its relevance to incidents of thrombosis

BACKGROUND

The contribution of moderate coagulation factor XII (FXII) deficiency to development of thromboembolism is still undetermined. We have tried to show the relevance of FXII deficiency to incidences of venous thrombosis by exploring the prevalence of F12 gene mutations in Chinese patients with thrombotic disorders.

METHODS

One hundred and six patients with venous thromboembolism (VTE) and 220 healthy controls were enrolled in study. The coding region and flanking sequences of F12 gene were amplified and sequenced to identify genetic variances. Patients with F12 mutations were also screened for other thrombotic risk factors.

RESULTS

Heterozygous F12 gene mutations were identified in 6 individuals with VTE and 10 healthy controls. Q336X and R66W were found in two healthy individuals; D291E was identified in a patient with DVT; and A343P was a recurrent mutation with a prevalence of 4.7% (5/106) in patient group and 3.6%(8/220) in healthy control. The prevalence of heterozygous mutations between the two groups had no significant difference. The association of A343P mutations with VTE was weak with an OR of 1.31 (95% CI 0.42-4.11). No other thrombophilia risk factors screened were positive in patients harboring heterozygous F12 mutations.

CONCLUSIONS

There were conflicting theories about the relationship between FXII deficiency and thrombosis formation. Heterozygous F12 mutation decreases the plasma FXII activity approximately by half and cause moderate FXII deficiency. Although multiple mutations were identified in both groups, the link between F12 heterozygous mutation and development of thrombotic disorders is weak and further studies are warranted to clarify their relationship.

Coagulation Factor Levels in Non-Metastatic Colorectal Cancer Patients

There is evidence that high plasma levels of factor (F) VIII, FIX, FXI and fibrinogen are independent risk factors for venous thromboembolism.

Aim

To determine the plasma concentrations of several coagulation factors and C4b-binding protein (C4BP) in a group of patients with non-metastatic colorectal cancer in order to investigate some aspects of cancer-acquired thrombophilia. Methods: Plasma fibrinogen, FII, FV, FVII, FVIII, FIX, FX, FXI and FXII activity levels and C4BP concentrations were determined in 73 patients with non-metastatic colorectal cancer (48 colon and 25 rectum) and in 67 matched control subjects. No one in either group had had previous thrombotic events.

Results

Mean plasma concentrations of fibrinogen (functional and antigen), FVIII, FIX, FV and C4BP were significantly higher in colorectal cancer patients than in control subjects, while FVII and FXII levels were significantly decreased. Several correlations were found between the increased coagulation factors and C4BP concentrations, while FVII was highly correlated with FXII.

Conclusions

In colorectal cancer patients high plasma fibrinogen, FVIII and FIX levels might represent further risk factors for venous thrombotic complications in the immediate post-surgery period, while decreased FVII and FXII concentrations may be an index of intravascular coagulation activation, still in a subclinical phase.

Inhibition of Factors XI and XII for Prevention of Thrombosis Induced by Artificial Surfaces

Exposure of blood to a variety of artificial surface induces contact activation, a process that contributes to the host innate response to foreign substances. On the foreign surface, the contact factors, factor XII (FXII), and plasma prekallikrein undergo reciprocal conversion to their fully active protease forms (FXIIa and α-kallikrein, respectively) by a process supported by the cofactor high-molecular-weight kininogen. Contact activation can trigger blood coagulation by conversion of factor XI (FXI) to the protease FXIa. There is interest in developing therapeutic inhibitors to FXIa and FXIIa because these activated factors can contribute to thrombosis in certain situations. Drugs targeting these proteases may be particularly effective in thrombosis triggered by exposure of blood to the surfaces of implantable medical devices. Here, we review clinical data supporting roles for FXII and FXI in thrombosis induced by medical devices, and preclinical data suggesting that therapeutic targeting of these proteins may limit surface-induced thrombosis.

The Intrinsic Pathway of Coagulation as a Target for Antithrombotic Therapy

Plasma coagulation in the activated partial thromboplastin time assay is initiated by sequential activation of coagulation factors XII, XI, and IX. While this series of proteolytic reactions is not an accurate model for hemostasis in vivo, there is mounting evidence that factor XI and factor XII contribute to thrombosis, and that inhibiting them can produce an antithrombotic effect with a small effect on hemostasis. This article discusses the contributions of components of the intrinsic pathway to thrombosis in animal models and humans, and results of early clinical trials of drugs targeting factors IX, XI, and XII.

Factor XI and factor XII as targets for new anticoagulants

Although the non-vitamin antagonist oral anticoagulants produce less intracranial bleeding than warfarin, serious bleeding still occurs. Therefore, the search for safer anticoagulants continues. Factor XII and factor XI have emerged as promising targets whose inhibition has the potential to prevent thrombosis with little or no disruption of hemostasis. Thus, thrombosis is attenuated in mice deficient in factor XII or factor XI and patients with congenital factor XII deficiency do not bleed and those with factor XI deficiency rarely have spontaneous bleeding. Strategies targeting factor XII and XI include antisense oligonucleotides to decrease their synthesis, inhibitory antibodies or aptamers, and small molecule inhibitors. These strategies attenuate thrombosis in various animal models and factor XI knockdown with an antisense oligonucleotide in patients undergoing knee replacement surgery reduced postoperative venous thromboembolism to a greater extent than enoxaparin without increasing bleeding. Therefore, current efforts are focused on evaluating the efficacy and safety of factor XII and factor XI directed anticoagulant strategies.

Factors XI and XII as Targets for New Anticoagulants

Compared with vitamin K antagonists, the direct oral anticoagulants (DOACs) are simpler to administer and are associated with less intracranial bleeding. Nonetheless, even with the DOACs, bleeding still occurs and many patients with atrial fibrillation fail to receive anticoagulant thromboprophylaxis because of the fear of bleeding. Therefore, there is an urgent need for safer anticoagulants. Recent investigations into the biochemistry of hemostasis and thrombosis have identified new targets for development of novel anticoagulants. Using data from complementary sources, including epidemiological studies and investigations in various animal models, the contact pathway has emerged as a potential mediator of thrombosis that plays a minor part in hemostasis. Consequently, factor (F) XII of the contact system and FXI in the intrinsic pathway have been identified as potentially safer targets of anticoagulation than thrombin or FXa. However, further studies are needed to identify which is the better target for the appropriate indication. This review highlights the evidence for focusing on FXI and FXII and examines the novel approaches directed at these new targets. These emerging strategies should address current unmet medical needs and provide new avenues by which to improve anticoagulant therapy by reducing the risk of bleeding.

Safety, pharmacokinetics, and pharmacodynamics of avoralstat, an oral plasma kallikrein inhibitor: phase 1 study

BACKGROUND

Avoralstat is a potent small-molecule oral plasma kallikrein inhibitor under development for treatment of hereditary angioedema (HAE). This first-in-human study evaluated the safety, tolerability, pharmacokinetics, and pharmacodynamics of avoralstat.

METHODS

This double-blind, placebo-controlled, ascending-dose cohort trial evaluated avoralstat single doses of 50, 125, 250, 500, and 1000 mg and multiple doses up to 2400 mg daily (100, 200, 400, and 800 mg every 8 h [q8 h] up to 7 days).

RESULTS

Avoralstat (n = 71) was generally well tolerated with no signals for a safety concern; there were no serious adverse events (AEs) or discontinuations due to AEs, and compared to placebo (n = 18), no notable difference in AEs. Four moderate severity AEs were reported in two subjects; syncope after a single 250 mg dose (one subject) and abdominal pain, back pain, and eczema after multiple doses of 800 mg avoralstat (one subject). For multiple-dose cohorts, the incidence of gastrointestinal AEs was highest at the 2400 mg/day dose. Elimination of avoralstat was bi-exponential with a terminal half-life of 12-31 h. Inhibition of plasma kallikrein was observed at all doses, and the degree of inhibition was highly correlated with avoralstat concentrations (R = 0.93). Mean avoralstat concentrations at doses ≥400 mg q8 h met or exceeded plasma kallikrein EC₅₀ values throughout the dosing interval.

CONCLUSION

Avoralstat was well tolerated, and drug exposure was sufficient to meet target levels for inhibition of plasma kallikrein. Based on these results, the 400 mg q8 h dose was selected for further evaluation in patients with HAE.

Emerging anticoagulant strategies

Despite the introduction of direct oral anticoagulants (DOACs), the search for more effective and safer antithrombotic strategies continues. Better understanding of the pathogenesis of thrombosis has fostered 2 new approaches to achieving this goal. First, evidence that thrombin may be as important as platelets to thrombosis at sites of arterial injury and that platelets contribute to venous thrombosis has prompted trials comparing anticoagulants with aspirin for secondary prevention in arterial thrombosis and aspirin with anticoagulants for primary and secondary prevention of venous thrombosis. These studies will help identify novel treatment strategies. Second, emerging data that naturally occurring polyphosphates activate the contact system and that this system is critical for thrombus stabilization and growth have identified factor XII (FXII) and FXI as targets for new anticoagulants that may be even safer than the DOACs. Studies are needed to determine whether FXI or FXII is the better target and to compare the efficacy and safety of these new strategies with current standards of care for the prevention or treatment of thrombosis. Focusing on these advances, this article outlines how treatment strategies for thrombosis are evolving and describes the rationale and approaches to targeting FXII and FXI. These emerging anticoagulant strategies should address unmet needs and reduce the systemic underuse of anticoagulation because of the fear of bleeding.

Thrombotic Events in Asymptomatic FXII Deficiency versus Symptomatic FXI Deficiency: Surprising Observations

OBJECTIVE

To evaluate the impact of an asymptomatic congenital clotting defect (FXII deficiency) versus that of a similar but symptomatic defect (FXI deficiency) on protection from thrombosis.

PATIENTS AND METHODS

All patients with FXII or FXI deficiency and thrombosis were gathered from a time-unlimited PubMed search that was carried out twice and from personal records. Combined defects were excluded. The defect had to be proven by the demonstration of a suited hereditary pattern and by a specific clotting assay. Only patients with a factor activity level of <30% of normal were selected.

RESULTS

Twenty-eight patients with an FXII deficiency presented with arterial thrombosis, mainly myocardial infarction, and 29 showed venous thrombosis; for FXI deficiency, these figures were 43 and 10, respectively. The ratio of arterial and venous thrombosis was 0.96 and 4.3, respectively, for FXII and FXI deficiency.

CONCLUSIONS

Factor FXII deficiency supplies no protection from arterial or venous thrombosis. FXI deficiency shows no protection from arterial thrombosis but appears to guarantee protection from venous thrombosis. A symptomatic, namely bleeding, condition (FXI deficiency) provides protection from venous thrombosis whereas an asymptomatic one (FXII deficiency) does not.

Myocardial Infarction and Arterial Thrombosis in Severe (Homozygous) FXII Deficiency: No Apparent Causative Relation

Twenty-one patients (12 female and 9 male) with severe (homozygous) factor XII (FXII) deficiency and 58 (32 female and 26 male) with heterozygous FXII deficiency were observed for an average 16.2 years. No patient with homozygous FXII deficiency experienced myocardial infarction or any other arterial thrombosis. The same was true for heterozygotes. The cases of FXII deficiency and arterial thrombosis reported in the literature were evaluated. In every instance, associated risk factors were present that could justify the arterial thrombosis. Dyslipidemia, hypertension, smoking, and diabetes mellitus were the most frequent findings. The examination of the few papers that dealt with the prevalence of arterial thrombosis in patients with severe FXII deficiency showed that only 1 patient of 61 experienced myocardial infarction. In conclusion, it seems that the role of FXII deficiency in the pathogenesis of arterial thrombosis is minor.

Thrombosis-free Surgical Procedures in Severe(Homozygote) Factor XII Deficiency: Report of Four Additional Cases and Literature Review

The outcome of various surgical procedures carried out in patients with severe (homozygote) factor XII deficiency were investigated for the appearance of blood coagulation-related complications with particular emphasis on thrombotic complications. The surgical procedures were total mastectomy, tonsillectomy and adenoidectomy, placement of a hip prosthesis, and double hernia repair. None of the patients slowed any complication. Several other reported cases of surgical procedures carried out in several patients ware found in the literature. Bleeding or thrombotic complications were noted in none of these cases. The surgical procedures in some cases were minor such as adenoidectomy, tonsillectomy, or nasal polyp removal. However several major surgical procedures were carried out in some patients (cholecystectomy, gastrectomy, repair of atrial septal defect, coronary bypass). All patients remained asymptomatic. In some cases whole blood and/or plasma were used as requested by the caring surgeons. In a few patients, the plasma was given prophylactically because of the long partial thromboplastin time. Finally, three patients (two for cardiac surgery and one after hip replacement) received heparin prophylaxis as foreseen by accepted procedures without the undue sequels. These data supply further evidence that factor XII deficiency does not only show any bleeding tendency but also can withstand even major surgical procedures without thrombotic complications.

Factor XI and contact activation as targets for antithrombotic therapy

The most commonly used anticoagulants produce therapeutic antithrombotic effects either by inhibiting thrombin or factor Xa (FXa) or by lowering the plasma levels of the precursors of these key enzymes, prothrombin and FX. These drugs do not distinguish between thrombin generation contributing to thrombosis from thrombin generation required for hemostasis. Thus, anticoagulants increase bleeding risk, and many patients who would benefit from therapy go untreated because of comorbidities that place them at unacceptable risk for hemorrhage. Studies in animals demonstrate that components of the plasma contact activation system contribute to experimentally induced thrombosis, despite playing little or no role in hemostasis. Attention has focused on FXII, the zymogen of a protease (FXIIa) that initiates contact activation when blood is exposed to foreign surfaces, and FXI, the zymogen of the protease FXIa, which links contact activation to the thrombin generation mechanism. In the case of FXI, epidemiologic data indicate this protein contributes to stroke and venous thromboembolism, and perhaps myocardial infarction, in humans. A phase 2 trial showing that reduction of FXI may be more effective than low molecular weight heparin at preventing venous thrombosis during knee replacement surgery provides proof of concept for the premise that an antithrombotic effect can be uncoupled from an anticoagulant effect in humans by targeting components of contact activation. Here, we review data on the role of FXI and FXII in thrombosis and results of preclinical and human trials for therapies targeting these proteins.

Decreased plasma levels of activated factor VII in patients with deep vein thrombosis

BACKGROUND

The initiating trigger in the development of deep vein thrombosis (DVT) remains unidentified. It has been suggested that tissue factor (TF)-bearing microparticles play a key role, which indicates a role for the TF pathway in the initiation of DVT.

OBJECTIVE

To assess the role of the TF pathway in the initiation of venous thrombosis, we measured plasma levels of factor VII and VIIa in patients with acute DVT and in controls.

METHODS

We included 148 patients diagnosed with acute DVT and 179 controls in this study. Antigen levels of FVII and FVIIa were measured by using assays recently developed in our laboratory.

RESULTS

Median FVII levels in patients were 109.8% (interquartile range [IQR] 86.0-153.2) compared with 102.2% (IQR 76.1-141.7) in controls. Individuals with FVII levels in the upper quartile had a 1.6-fold increased risk for the presence of a DVT (odds ratio 1.6, 95% confidence interval 0.8-3.1). Median FVIIa levels in patients were 50.2 ng mL(-1) (IQR 25.2-86.1) compared with 96.6 ng mL(-1) (69.9-168.9) in controls. Individuals with FVIIa levels in the lowest quartile had a > 5-fold increased risk for the presence of a DVT (odds ratio 5.5, 95% confidence interval 2.8-10.6). Both risks did not change substantially after adjustment for potential confounders.

CONCLUSION

Decreased plasma levels of FVIIa in patients with deep vein thrombosis may indicate ongoing consumption of FVIIa and suggest a contributory role for TF in venous thrombus formation.

Epidemiologic and clinical data linking factors XI and XII to thrombosis

Currently available evidence supports the contention that elevated levels of factor XI (fXI) are associated with a greater risk of venous thromboembolism and ischemic stroke, but, less convincingly, with myocardial infarction. Conversely, reduced plasma levels of fXI seem to offer some protection from venous thromboembolism and stroke, but not myocardial infarction. Factor XI-deficient patients are at risk for certain types of bleeding, particularly posttraumatic hemorrhage on mucosal surfaces where there is a high endogenous fibrinolytic activity. In contrast, the situation with fXII in human thrombosis remains enigmatic. Deficiency of fXII is clearly not associated with any bleeding risk, but neither does it seem to be protective against thrombosis. The longstanding debate as to whether partial fXII deficiency represents a risk factor for thrombosis remains unresolved, with seemingly conflicting results depending on study design, type of assay used, and analyte evaluated. The possibility that elevated fXII levels represent a risk factor for thrombosis is not borne out in the literature.

Coagulation and the vessel wall in pulmonary embolism

Venous thromboembolism comprises deep-vein thrombosis, thrombus in transit, acute pulmonary embolism, and chronic thromboembolic pulmonary hypertension (CTEPH). Pulmonary thromboemboli commonly resolve, with restoration of normal pulmonary hemodynamics. When they fail to resorb, permanent occlusion of the deep veins and/or CTEPH are the consequences. Apart from endogenous fibrinolysis, venous thrombi resolve by a process of mechanical fragmentation, through organization of the thromboembolus by invasion of endothelial cells, leukocytes, and fibroblasts leading to recanalization. Recent data utilizing various models have contributed to a better understanding of venous thrombosis and the resolution process that is directed at maintaining vascular patency. This review summarizes the plasmatic and cellular components of venous thrombus formation and resolution.

Factor XII: a drug target for safe interference with thrombosis and inflammation

Data from experimental animal models revealed an essential role for factor XII (FXII) in thrombotic occlusive diseases. In contrast to other blood coagulation factors, deficiency in the protease is not associated with abnormal bleeding from injury sites (hemostasis) in patients or in animals. Cumulatively, these findings suggest that FXII could be targeted as a new method of anticoagulation that is devoid of bleeding risks. An FXIIa-neutralizing antibody, 3F7, has been developed that inhibited thrombosis in an extracorporeal membrane oxygenation (ECMO) system as efficiently as heparin. However, in sharp contrast to heparin, 3F7 treatment was not associated with an increase in therapy-associated hemorrhage. In this review, we summarize current knowledge of FXII physiology and pharmacology.

Role of Factor XII in hemostasis and thrombosis: clinical implications

The plasma coagulation system reacts quickly to limit blood loss from injury sites but also contributes to vascular thrombosis. In current models of hemostatic balance, normal coagulation and thrombosis represent two sides of the same coin, however, recent data from gene-deleted murine models have challenged this dogma. Deficiency of coagulation Factor XII (Hageman factor), a serine protease that initiates the intrinsic pathway of coagulation, severely impairs arterial thrombus formation but is not associated with excessive bleeding. These findings suggest that fibrin-generating mechanisms that operate during pathologic thrombus formation involve pathways distinct from those that are active during normal hemostasis. As Factor XII selectively contributes to thrombus formation in occlusive disease, but not to normal hemostasis, inhibition of this protease may offer a novel treatment strategy for prevention of arterial thrombosis with minimal or no risk of bleeding.

Factor XII inhibition reduces thrombus formation in a primate thrombosis model

The plasma zymogens factor XII (fXII) and factor XI (fXI) contribute to thrombosis in a variety of mouse models. These proteins serve a limited role in hemostasis, suggesting that antithrombotic therapies targeting them may be associated with low bleeding risks. Although there is substantial epidemiologic evidence supporting a role for fXI in human thrombosis, the situation is not as clear for fXII. We generated monoclonal antibodies (9A2 and 15H8) against the human fXII heavy chain that interfere with fXII conversion to the protease factor XIIa (fXIIa). The anti-fXII antibodies were tested in models in which anti-fXI antibodies are known to have antithrombotic effects. Both anti-fXII antibodies reduced fibrin formation in human blood perfused through collagen-coated tubes. fXII-deficient mice are resistant to ferric chloride-induced arterial thrombosis, and this resistance can be reversed by infusion of human fXII. 9A2 partially blocks, and 15H8 completely blocks, the prothrombotic effect of fXII in this model. 15H8 prolonged the activated partial thromboplastin time of baboon and human plasmas. 15H8 reduced fibrin formation in collagen-coated vascular grafts inserted into arteriovenous shunts in baboons, and reduced fibrin and platelet accumulation downstream of the graft. These findings support a role for fXII in thrombus formation in primates.

Inhibition of vascular permeability by antisense-mediated inhibition of plasma kallikrein and coagulation factor 12

Hereditary angioedema (HAE) is a rare disorder characterized by recurrent, acute, and painful episodes of swelling involving multiple tissues. Deficiency or malfunction of the serine protease inhibitor C1 esterase inhibitor (C1-INH) results in HAE types 1 and 2, respectively, whereas mutations in coagulation factor 12 (f12) have been associated with HAE type 3. C1-INH is the primary inhibitor of multiple plasma cascade pathways known to be altered in HAE patients, including the complement, fibrinolytic, coagulation, and kinin-kallikrein pathways. We have selectively inhibited several components of both the kinin-kallikrein system and the coagulation cascades with potent and selective antisense oligonucleotides (ASOs) to investigate their relative contributions to vascular permeability. We have also developed ASO inhibitors of C1-INH and characterized their effects on vascular permeability in mice as an inducible model of HAE. Our studies demonstrate that ASO-mediated reduction in C1-INH plasma levels results in increased vascular permeability and that inhibition of proteases of the kinin-kallikrein system, either f12 or prekallikrein (PKK) reverse the effects of C1-INH depletion with similar effects on both basal and angiotensin converting enzyme (ACE) inhibitor-induced permeability. In contrast, inhibition of coagulation factors 11 (f11) or 7 (f7) had no effect. These results suggest that the vascular defects observed in C1-INH deficiency are dependent on the kinin-kallikrein system proteases f12 and PKK, and not mediated through the coagulation pathways. In addition, our results highlight a novel therapeutic modality that can potentially be employed prophylactically to prevent attacks in HAE patients.

Evaluation of the Q analyzer, a new cap-piercing fully automated coagulometer with clotting, chromogenic, and immunoturbidometric capability

The Q analyzer is a recently launched fully automated photo-optical analyzer equipped with primary tube cap-piercing and capable of clotting, chromogenic, and immunoturbidometric tests. The purpose of the present study was to evaluate the performance characteristics of the Q analyzer with reagents from the instrument manufacturer. We assessed precision and throughput when performing coagulation screening tests, prothrombin time (PT)/international normalized ratio (INR), activated partial thromboplastin time (APTT), and fibrinogen assay by Clauss assay. We compared results with established reagent instrument combinations in widespread use. Precision of PT/INR and APTT was acceptable as indicated by total precision of around 3%. The time to first result was 3  min for an INR and 5  min for PT/APTT. The system produced 115 completed samples per hour when processing only INRs and 60 samples (120 results) per hour for PT/APTT combined. The sensitivity of the DG-APTT Synth/Q method to mild deficiency of factor VIII (FVIII), IX, and XI was excellent (as indicated by APTTs being prolonged above the upper limit of the reference range). The Q analyzer was associated with high precision, acceptable throughput, and good reliability. When used in combination with DG-PT reagent and manufacturer's instrument-specific international sensitivity index, the INRs obtained were accurate. The Q analyzer with DG-APTT Synth reagent demonstrated good sensitivity to isolated mild deficiency of FVIII, IX, and XI and had the advantage of relative insensitivity to mild FXII deficiency. Taken together, our data indicate that the Q hemostasis analyzer was suitable for routine use in combination with the reagents evaluated.

In vivo roles of factor XII

Coagulation factor XII (FXII, Hageman factor, EC = 3.4.21.38) is the zymogen of the serine protease, factor XIIa (FXIIa). FXII is converted to FXIIa through autoactivation induced by "contact" to charged surfaces. FXIIa is of crucial importance for fibrin formation in vitro, but deficiency in the protease is not associated with excessive bleeding. For decades, FXII was considered to have no function for coagulation in vivo. Our laboratory developed the first murine knockout model of FXII. Consistent with their human counterparts, FXII(-/-) mice have a normal hemostatic capacity. However, thrombus formation in FXII(-/-) mice is largely defective, and the animals are protected from experimental cerebral ischemia and pulmonary embolism. This murine model has created new interest in FXII because it raises the possibility for safe anticoagulation, which targets thrombosis without influence on hemostasis. We recently have identified platelet polyphosphate (an inorganic polymer) and mast cell heparin as in vivo FXII activators with implications on the initiation of thrombosis and edema during hypersensitivity reactions. Independent of its protease activity, FXII exerts mitogenic activity with implications for angiogenesis. The goal of this review is to summarize the in vivo functions of FXII, with special focus to its functions in thrombosis and vascular biology.

Inhibition of factor XI activation attenuates inflammation and coagulopathy while improving the survival of mouse polymicrobial sepsis

Severe bacterial sepsis often leads to a systemic procoagulant and proinflammatory condition that can manifest as disseminated intravascular coagulation, septic shock, and multiple organ failure. Because activation of the contact proteases factor XII (FXII), prekallikrein, and factor XI (FXI) can trigger coagulation and inflammatory responses, the contact factors have been considered potential targets for the treatment of sepsis. However, the pathogenic role of contact activation in severe infections has not been well defined. We therefore investigated whether an anticoagulant antibody (14E11) that selectively inhibits prothrombotic FXI activation by activated FXII (FXIIa) modifies the course of bowel perforation-induced peritoneal sepsis in mice. Early anticoagulation with 14E11 suppressed systemic thrombin- antithrombin complex formation, IL-6, and TNF-α levels, and reduced platelet consumption in the circulation and deposition in the blood vessels. Treatment with 14E11 within 12 hours after bowel perforation significantly improved survival compared with vehicle treatment, and the saturating dose did not increase tail bleeding. These data suggest that severe polymicrobial abdominal infection induces prothrombotic FXI activation, to the detriment of the host. Systemic anticoagulation by inhibiting FXI activation or FXIIa procoagulant activity during sepsis may therefore limit the development of disseminated intravascular coagulation without increasing bleeding risks.

The many faces of the contact pathway and their role in thrombosis

Understanding inherent differences between thrombosis and hemostasis in the vascular system are critical to developing safe and effective anticoagulants. To this end, constituents of the contact activated and intrinsic pathway of coagulation appear to be involved in pathological thrombus formation, but are not required for normal hemostasis. In addition to coagulation, activation of the contact system is involved in fibrinolytic, inflammatory, and angiogenic processes that can also contribute to the thrombotic environment. This review discusses the role of the contact system in these processes, and highlights the potential of FXII and FXI as safer targets for antithrombotic therapy.

Factor XI and XII as antithrombotic targets

PURPOSE OF REVIEW

Arterial and venous thrombosis are major causes of morbidity and mortality, and the incidence of thromboembolic diseases increases as a population ages. Thrombi are formed by activated platelets and fibrin. The latter is a product of the plasma coagulation system. Currently available anticoagulants such as heparins, vitamin K antagonists and inhibitors of thrombin or factor Xa target enzymes of the coagulation cascade that are critical for fibrin formation. However, fibrin is also necessary for terminating blood loss at sites of vascular injury. As a result, anticoagulants currently in clinical use increase the risk of bleeding, partially offsetting the benefits of reduced thrombosis. This review focuses on new targets for anticoagulation that are associated with minimal or no therapy-associated increased bleeding.

RECENT FINDINGS

Data from experimental models using mice and clinical studies of patients with hereditary deficiencies of coagulation factors XI or XII have shown that both of these clotting factors are important for thrombosis, while having minor or no apparent roles in processes that terminate blood loss (hemostasis).

SUMMARY

Hereditary deficiency of factor XII (Hageman factor) or factor XI, plasma proteases that initiate the intrinsic pathway of coagulation, impairs thrombus formation and provides protection from vascular occlusive events, while having a minimal impact on hemostasis. As the factor XII-factor XI pathway contributes to thrombus formation to a greater extent than to normal hemostasis, pharmacological inhibition of these coagulation factors may offer the exciting possibility of anticoagulation therapies with minimal or no bleeding risk.

Selective depletion of plasma prekallikrein or coagulation factor XII inhibits thrombosis in mice without increased risk of bleeding

Recent studies indicate that the plasma contact system plays an important role in thrombosis, despite being dispensable for hemostasis. For example, mice deficient in coagulation factor XII (fXII) are protected from arterial thrombosis and cerebral ischemia-reperfusion injury. We demonstrate that selective reduction of prekallikrein (PKK), another member of the contact system, using antisense oligonucleotide (ASO) technology results in an antithrombotic phenotype in mice. The effects of PKK deficiency were compared with those of fXII deficiency produced by specific ASO-mediated reduction of fXII. Mice with reduced PKK had ∼ 3-fold higher plasma levels of fXII, and reduced levels of fXIIa-serpin complexes, consistent with fXII being a substrate for activated PKK in vivo. PKK or fXII deficiency reduced thrombus formation in both arterial and venous thrombosis models, without an apparent effect on hemostasis. The amount of reduction of PKK and fXII required to produce an antithrombotic effect differed between venous and arterial models, suggesting that these factors may regulate thrombus formation by distinct mechanisms. Our results support the concept that fXII and PKK play important and perhaps nonredundant roles in pathogenic thrombus propagation, and highlight a novel, specific and safe pharmaceutical approach to target these contact system proteases.

Novel deleterious mutation in the F12 gene in a Korean family with severe coagulation factor XII deficiency

Coagulation factor XII (FXII) is involved in the initiation of blood coagulation, fibrinolysis, complement systems, and bradykinin generation. Hereditary deficiency of FXII is caused by mutations in the F12 gene. In this report, we describe a Korean family with severe FXII deficiency from F12 mutations. The proband was a 46-year-old woman and was shown to have a markedly prolonged activated partial thromboplastin time at 126.7 s (reference range, 29-42 s) on routine health checkup. She had no history of bleeding tendency. Complete correction of prolonged activated partial thromboplastin time on mixing test prompted us to perform factor assays, which revealed a markedly decreased FXII activity (<0.5%; below the detection limit). Direct sequencing analyses for F12 showed that the proband was compound heterozygous for two deleterious mutations, c.249delG (p.Q83HfsX12) and c.405C>A (p.C135X). Family study showed that her sister with prolonged activated partial thromboplastin time at 83.8 s and FXII activity less than 0.5% was also compound heterozygous for the mutations. Q83HfsX12 was a novel frameshift mutation in exon 4 of F12, and C135X is a nonsense mutation previously reported in a Korean patient who was homozygous for the mutation. Thus, the C135X mutation is a recurrent mutation in Korean individuals with FXII deficiency, potentially with a founder effect.

The factor XII -4C>T variant and risk of common thrombotic disorders: A HuGE review and meta-analysis of evidence from observational studies

Coagulation factor XII is involved in thrombus formation and therefore may play a role in the etiology of thrombotic disorders. A common variant in the factor XII (F12) gene (-4C>T, rs1801020) results in decreased plasma levels of this coagulation factor. The existence of associations between low factor XII levels or F12 variants and thrombotic outcomes has been debated for more than a decade. The authors conducted a review and meta-analysis to evaluate the evidence for an association between F12 -4C>T and 2 common thrombotic outcomes: venous thromboembolism and myocardial infarction, which are hypothesized to share some etiologic pathways. MEDLINE, EMBASE, and HuGE Navigator were searched through July 2009 to identify relevant epidemiologic studies, and data were summarized using random-effects meta-analysis. Sixteen candidate gene studies (4,386 cases, 40,089 controls) were analyzed. None of the investigated contrasts reached statistical significance at P < 0.05, apart from a very weak association with myocardial infarction for the TT + CT versus CC contrast (odds ratio = 1.13, 95% confidence interval: 1.00, 1.27). Overall, based on the synthesis of observational studies, the evidence for an association between F12 -4C>T and venous thromboembolism and myocardial infarction is weak.

Intrinsic clotting factors in dependency of age, sex, body mass index, and oral contraceptives: definition and risk of elevated clotting factor levels

Elevated clotting factors have been demonstrated to be a risk factor for venous thromboembolism (VTE). The aim of our study was to investigate the impact of age, sex, body mass index, and oral contraceptives on the clotting factor activities of factors VIII, IX, XI, and XII and their impact on the cutoff definition and risk of VTE associated with elevated clotting factors. Factor VIII, IX, XI, and XII activities were measured in 499 blood donors and 286 patients with VTE. Age and body mass index predicted significantly and independently the clotting factor activities of factors VIII, IX, and XI, whereas use of oral contraceptives predicted factor IX, XI, and XII levels. Percentiles of clotting factor activities, which are often used for the cutoff definition of elevated clotting factors, varied due to the effect of age, body mass index, and oral contraceptives. The adjusted odds ratios for VTE were 10.3 [95% confidence interval (CI) 5.1-20.7], 6.1 (95% CI 3.1-12.0), and 3.3 (95% CI 1.9-5.8) for elevated factors VIII, IX, and XI, respectively. Furthermore, our study demonstrates for the first time that elevated factor XII is associated with an increased risk of VTE (adjusted odds ratio 2.9, 95% CI 1.6-5.3).

Hageman factor, platelets and polyphosphates: early history and recent connection

  Platelet activation and blood coagulation are essential for hemostasis and contribute to a variety of other biological processes such as inflammation, complement activation and tissue repair. Factor (F)XII, originally called Hageman factor, plays an important role in the kallikrein-kinin system by activating prekallikrein. In the 1960s, a platelet activity that promoted FXII activation was identified but its biochemical nature remained unknown. Inorganic polyphosphates (poly P) are polymers that consist of many phosphate residues linked by phosphoanhydride bonds. These polymers exist in all living organisms. In bacteria, poly P is important for growth and survival. Recently, poly P has been identified in human platelet dense granules. Studied have shown that upon platelet activation and secretion, poly P activates FXII, indicating that it is most likely the elusive platelet FXII activator. Poly P also regulates coagulation and fibrinolysis. In this review, we focus on early studies of FXII and the identification of platelet FXII activation activity, and discuss recent findings of poly P in FXII activation and coagulation.

Factor XII: what does it contribute to our understanding of the physiology and pathophysiology of hemostasis & thrombosis

Factor XII (FXII) is a coagulation protein that is essential for surface-activated blood coagulation tests but whose deficiency is not associated with bleeding. For over forty years, investigators in hemostasis have not considered FXII important because its deficiency is not associated with bleeding. It is because there is a dichotomy between abnormal laboratory assay findings due to FXII deficiency and clinical hemostasis that investigators sought explanations for physiologic hemostasis independent of FXII. FXII is a multidomain protein that contains two fibronectin binding consensual sequences, two epidermal growth factor regions, a kringle region, a proline-rich domain, and a catalytic domain that when proteolyzed turns into a plasma serine protease. Recent investigations with FXII deleted mice that are protected from thrombosis indicate that it contributes to the extent of developing thrombus in the intravascular compartment. These findings suggest that it has a role in thrombus formation without influencing hemostasis. Last, FXII has been newly appreciated to be a growth factor that may influence tissue injury repair and angiogenesis. These combined studies suggest that FXII may become a pharmacologic target to reduce arterial thrombosis risk and promote cell repair after injury, without influencing hemostasis.

Coagulation factors IX through XIII and the risk of future venous thrombosis: the Longitudinal Investigation of Thromboembolism Etiology

Higher levels of procoagulant factors and factor XII deficiency may be risk factors for first venous thromboembolism (VTE). We studied associations of coagulation factors IX through XIII with risk of future VTE in 2 general population samples. Using a nested case-control study combining the 21 860 participants of the Atherosclerosis Risk in Communities study and the Cardiovascular Health Study, we determined antigenic levels of these coagulation factors in primarily pre-event blood samples from 462 participants who subsequently developed VTE and 1047 participants who remained free of VTE. Only elevated levels of factors IX and XI were associated with increased risk of VTE after adjustment for age, sex, race, and study. For factor IX, the odds ratio (OR) was 1.4 (95% confidence interval [CI], 1.0-2.0) comparing the top to bottom quintile. The OR for factor XI was higher: 2.0 (95% CI, 1.4-2.9). With further adjustment for body mass index and diabetes, only elevated factor XI remained associated with VTE risk: OR 1.8 (95% CI, 1.3-2.7). Associations were similar by study and whether the thrombosis was idiopathic or secondary. Factor XII deficiency was not related to VTE risk. Among these procoagulant factors, only elevated factor XI was a risk factor for VTE.